1. Field of the Invention
This invention relates to the field of plasma etching of silicon, particularly in the fabrication of semiconductor devices.
2. Prior Art
In the fabrication of semiconductor devices using monocrystalline silicon substrates, it is often desired to etch certain areas of the silicon substrate, particularly when manufacturing high packing density integrated circuit chips. In the prior art this has been accomplished by the use of plasma etching. In the plasma etching process, the silicon is first coated with a layer of photoresist. Openings are formed in the photoresist to expose selected areas of the underlying silicon. The silicon is then placed in an etch chamber. The chamber, which is a controlled environment, includes means for introducing gas to the chamber and electrodes for producing plasma from the gas. Typically chlorinated gases such as C12, CC14, etc. are used to form plasmas to etch the silicon. The gas is introduced to the chamber and RF energy is applied to produce the plasma. Etching takes place until the RF energy is removed, with the timing of the process, pressure, RF energy and flow rate controlling the depth of etching.
There are several disadvantages with etching silicon by the prior art methods. First, the chlorinated gases used are highly hazardous to human life. For example, carbon tetrachloride, CC14, is toxic to humans and has a permissible exposure rate of only 10 parts per million (ppm). At levels above 300 ppm, there is an immediate threat to human life. In addition, CC14 is a suspected carcinogen. By itself, chlorine (C12) is toxic and corrosive with a permissible exposure limit as defined by the Occupational Safety and Health Administration (OSHA) of only 1 ppm. Immediate danger to life begins at 25 ppm. As a result, expensive precautions must be taken to prevent leaks of the gas which could endanger human life. These precautions add to the expense of the etching process.
Secondly, the chlorine gas is highly corrosive, resulting in shorter lifetimes of components exposed to the gas.
Third, during the etching process of the silicon, the chlorine gas damages the surface of the silicon, leaving it rough. As a result, the formation of oxide layers is adversely affected. This leads to undependable device characteristics. This damage can be cured, but only with an additional thermal step.
Finally, the chlorine gas can etch the silicon isotropically, undercutting the photoresist layer and requiring larger design windows to obtain acceptable devices.
Therefore, it is an object of the present invention to provide a method of etching silicon which does not involve hazardous materials.
It is a further object of the present invention to provide a method of etching silicon which does not use corrosive materials.
Yet another object of the present invention is a method of etching silicon which does not damage the surface of the silicon.
A still further object of the present invention is a method of etching silicon anisotropically to eliminate undercutting of the photoresist layer.